A normal heartbeat involves generation of an electrical impulse and propagation of the electrical impulse across the heart, which causes each chamber of the heart to appropriately contract. Sometimes aberrant conductive pathways develop in heart tissues, and these disrupt the normal path of the electrical impulse. For example, anatomical obstacles or conduction blocks in heart tissue can disrupt the normal propagation of an impulse by causing the impulse to degenerate into several circular wavelets that circulate about the obstacles, thus disrupting normal activation within the heart tissue and chambers. Slow conduction zones in animal and human hearts constrained by anatomical or conduction blocks are believed to exist. Such a zone is a localized region of the heart tissue which propagates an impulse at a slower speed than normal heart tissue thus sometimes resulting in errant, circular propagation patterns or reentrant pathways. Reentrant pathways can result in an arrhythmia from the re-excitation of a region of cardiac tissue by a single impulse, and the arrhythmia continues for one or more cycles and sometimes results in tachycardia. Re-entrant ventricular tachycardia (VT) is an abnormally rapid ventricular rhythm with aberrant ventricular excitation (wide QRS complexes), usually in excess of 150 per minute, which is generated within the ventricle of the heart as a result of a reentrant pathway.
To treat VT, it is desirable first to determine the physical location of the source(s) of the aberrant pathways. Once located, the heart tissue can be destroyed by heat, chemicals, RF ablation, and/or other means. Ablation can remove the aberrant conductive pathway, restoring normal myocardial contraction. More specifically, to treat VT, the slow conduction zone must be located and destroyed or partially destroyed, with the goal of eliminating the slow conduction zone's ability to conduct electrical impulses.
It is known for physicians to examine the propagation of electrical impulses in heart tissue to locate aberrant conductive pathways. The techniques used to analyze these pathways, commonly called "mapping," identify tissue which can be ablated to treat the arrhythmia.
One form of conventional cardiac tissue mapping techniques uses multiple electrodes positioned in contact with epicardial heart tissue to obtain multiple electrograms. The physician stimulates myocardial tissue by introducing pacing signals through one or more (e.g., a pair) of electrodes and visually observes the morphologies of the electrograms recorded during pacing to determine activation times of electrograms at various epicardial locations. ("Pacing" means artificially stimulating the heart with one or more electrical signals.) Electrogram morphology refers to the shape of the electrical signals recorded from electrodes placed in the heart. This conventional mapping technique requires invasive open heart surgical techniques to position the electrodes on the epicardial surface of the heart. Furthermore, conventional cardiac tissue mapping techniques used for detecting local electrical events in heart tissues are often unable to interpret electrograms with multiple morphologies. Such electrograms are encountered, for example, when mapping a heart undergoing ventricular tachycardia (VT). For these reasons, consistent identification of foci cannot be achieved with current multi-electrode mapping technologies.
An improvement to conventional multiple-electrode cardiac tissue mapping techniques is disclosed in U.S. Pat. No. 5,577,509 which is incorporated in its entirety by reference. A minimally invasive basket catheter or multi-electrode structure is used as the electrodes for pacing and monitoring the heart such that open heart surgery is not required. Despite this improvement, this technique is less than ideal because the technique for choosing the earliest activation times necessary to produce the isochronal displays has not been perfected.
Another form of conventional cardiac tissue mapping techniques, called pace mapping, uses a roving electrode in a heart chamber for pacing the heart at various endocardial locations. The heart is monitored during pacing and electrocardiograms that are produced are compared to electrocardiograms produced during VT. VT may have been induced or spontaneous. When a pacing signal is applied to a slow conduction zone, the excitation wavefront caused by the pacing signal gets caught in the same circular motion that results in the VT. Therefore, a large proportion of the electrocardiograms produced during pacing will have morphologies that match the electrograms recorded during VT. In searching for a slow conduction zone, the physician must visually compare all paced electrocardiograms to those previously recorded during VT. The physician must constantly relocate the roving electrode to a new location to systematically map the endocardium. This pace-mapping technique is complicated and time consuming. It requires repeated manipulation and movement of the pacing electrode. At the same time, it requires the physician visually to assimilate and interpret the electrocardiograms. Improvements to the conventional pace mapping procedure are described in U.S. Pat. No. 5,595,183 which is incorporated in its entirety by reference. This patent describes methods of automatically comparing the electrograms obtained during pacing at multiple sites to those taken during induced or spontaneous VT. This patent also describes methods to pace automatically at multiple sites in a sequence to identify rapidly and efficiently pacing sites that provide a good match of 12-lead ECGs obtained during pacing and during VT.
Entrainment mapping is another conventional cardiac tissue mapping technique used to identify potential ablation sites for curing VT. In this technique, VT is pace-induced, then pacing is initiated at cycle lengths a little shorter than the VT cycle length. In some cases, the beating of the heart can be captured by the pacing attempt. If the heart is successfully captured, the electrocardiograms (recorded by 12 lead body surface electrodes) can exhibit changes in the morphology of the QRS complex as the heart is captured. In other circumstances, there is little or no change in the QRS morphology. If the heart is captured without a change in QRS morphology, the result is called concealed entrainment. On the other hand, if the heart is captured with a change in electrocardiogram waveform, the result is described as entrainment with fusion. Successful ablation sites can be identified as they usually exhibit concealed entrainment, with some additional timing constraints. However, only about 30% of the ablation attempts at sites identified by this method cure the VT.